This led to the recommendation for the development of a unified approach for such investigations. These estimates of key epidemiological parameters are crucial to understand and predict the health impacts, inform policy and public health interventions at the local and national level.Ī systematic review and meta-analysis of within household transmission for 2009 pandemic influenza A(H1N1) found that estimates for household secondary infection rate/risk are highly heterogeneous.( 3) Heterogeneity in included HHTIs was at least in part attributed to varying household definitions, secondary case ascertainment and testing methods, and duration of follow up. Conducting HHTIs during the early stages of emergence of a novel respiratory pathogen provides an opportunity to swiftly characterise the transmissibility and severity of the pathogen. HHTIs facilitate the collection of important epidemiological, clinical, and virological data in well-defined closed settings, where households contacts can be more accurately ascertained and followed up to identify infector-infectee pairs than in the general population including both virological and serological evidence of infection.

Since the early stages of the COVID-19 pandemic, households have been a major setting of SARS-CoV-2 transmission.( 2) As such, household transmission investigations (HHTIs) – including studies of the First Few X cases (FFX) in household settings – provide an opportunity to explore transmission dynamics and severity of SARS-CoV-2. It was declared a Public Health Emergency of International Concern by the World Health Organization (WHO) on 30 January 2020 and characterised as a pandemic in March 2020.( 1) The large, unexplained variance in hSAR estimates emphasises the need for improved standardisation in planning, conduct and analysis, and for clear and comprehensive reporting of FFX and HHTIs, to guide evidence-based pandemic preparedness and response efforts for SARS-CoV-2, influenza and future novel respiratory viruses.Ĭoronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, was first reported in Wuhan, China, in December 2019. hSAR point estimates ranged from 2%–90% (95% prediction interval: 3%–71% I 2=99.7%) I 2 values remained >99% in subgroup analyses, indicating high, unexplained heterogeneity and leading to a decision not to report pooled hSAR estimates. Of 9988 records retrieved, 80 articles (64 from databases 16 provided by WHO Unity Studies collaborators) were retained in the systematic review and 62 were included in the primary meta-analysis. Heterogeneity was assessed by visually inspecting overlap of CIs on forest plots and quantified in meta-analyses. Values for hSAR and 95% confidence intervals (CIs) were extracted or calculated from crude data. We used a bespoke tool to assess investigation methodological quality. Standardised early results were shared by WHO Unity Studies Collaborators (to 1 October 2021). We searched Medline, Embase, Web of Science, Scopus and medRxiv/bioRxiv for ‘Unity-aligned’ FFX and HHTI published between 1 December 2019 and 26 July 2021. We conducted a systematic review and meta-analysis according to PRISMA 2020 guidelines (PROSPERO registration:CRD42021260065). We aimed to estimate the household secondary infection attack rate (hSAR) of SARS-CoV-2 in investigations aligned with the WHO Unity Studies HHTI protocol. First Few X cases (FFX) investigations and Household transmission investigations (HHTI) are essential epidemiological tools for early characterisation of novel infectious pathogens and their variants.

Ffx hi risk probability